Values

Is either a non-negative explicit <length> value or a keyword denoting the thickness of the border area of a box. The keyword must be one of the following values:

thin

A thin border

medium

A medium border

thick

A thick border

The specification doesn't precisely define the thickness of each of the keywords, which is therefore implementation specific. Nevertheless, it requests that the thickness does follow the thin ≤ medium ≤ thick inequality and that the values are constant on a single document.

inherit

Is a keyword indicating that all four values are inherited from their parent's element calculated value.